Low grade methane gas sources such as that arising from the decay of organic materials have been recognized as potential energy sources for at least 50 years. Such gas sources include gas from landfill sites and anaerobic digesters that produce "biogas" comprised primarily of methane and carbon dioxide. Numerous other trace impurities as well as oxygen and nitrogen may also be present in the biogas in varying amounts. Biogas escaping from landfill sites possesses both environmental and safety hazards. Further, both methane and carbon dioxide components in the biogas are potentially valuable products if properly purified. Hence, it would be advantageous to capture the energy value of the biogas while eliminating the environmental and safety hazards. In spite of the desirability of utilizing biogas from landfills and digesters, such methane gas sources have been underutilized because of problems effectively purifying the gas, namely, removing the trace amounts of noxious substances, and then effectively separating the carbon dioxide component from the methane component. About one third to one half of the biogas stream generated by anaerobic decay of organic material is carbon dioxide. Hence, the volumetric energy content of the unpurified biogas stream is substantially less than that of pipeline natural gas. Accordingly, unpurified biogas cannot be introduced into gas pipelines or easily utilized in conventional equipment without processing the gas mixture to remove the carbon dioxide and other impurities.
Numerous systems for purifying biogas sources have been suggested. Separation systems based on membranes, pressure swing adsorption, temperature swing adsorption, chemical absorption, and cryogenic processes have all been reported. Each of these systems has the potential of successfully purifying biogas at sites where large volumes of biogas are available for processing or where final methane purities below 95% are acceptable. However, none of these is economically viable for biogas sources smaller than one to two million standard cubic feet per day. For biogas sources producing less than this volume per day or where high purities are required, the capital investment, the operating costs, and or system complexity limit practical or economic use of existing systems.
The harsh, corrosive, continuous operating environments present at landfill sites limit the effectiveness of systems requiring maintenance, supervision, or chemical additives. Complex systems generally have higher capital and maintenance costs.
In principle, biogas can be cryogenically separated into its components using distillation techniques. Unfortunately, distillation techniques are more difficult for biogas mixtures of carbon dioxide and methane because of several unique features of the phases present in equilibrium mixtures. Cryogenic separations may be broadly divided into continuous and non-continuous (batch) approaches. Continuous cryogenic systems utilize a region or zone where carbon dioxide and methane are continuously separated from one another through the phase differences between components. For example, to obtain a purity of &gt;98% methane at a constant pressure below 700 psia, the solid CO.sub.2 that readily forms must be separated from the mixture feed stream. Operation below the critical point of the mixture is required to maintain distinct phases and allow phase separation. The range of temperature and pressure values available for such conventional cryogenic distillation is quite limited.
Numerous cryogenic processes for separating carbon dioxide and methane are taught in the prior art. For example, A. S. Holmes, et al. (U.S. Pat. No. 4,462,814) teach a process and apparatus for avoiding a solid carbon dioxide phase in a distillation process. Commonly termed the Ryan-Holmes process, alkane additives such as propane or butane are used to avoid solid CO.sub.2 formation during liquid distillation-based separation. The butane or propane is separated from the CO.sub.2 after co-distillation from the CH.sub.4 and is recirculated into the distillation tower. Heavy hydrocarbons (C3+) are added to the feedstream to allow operation with decreased pressures and elevated temperatures without solid CO.sub.2 formation. The addition of n-butane to the feedstream allows the distillation of the mixture to occur well within the liquid-vapor phase, eliminating solid CO.sub.2 formation in the distillation column. In addition, the critical pressure of the mixture is raised to create a greater range of acceptable operating pressures.
The Ryan-Holmes Process, however, has two significant limitations for biogas purification. First, the system complexity leads to high capital costs and the inability to scale to smaller feedstreams. As pointed out above, such costs are problematic in landfill recovery systems. Second, this process requires a supply of propane or heavier alkanes that are generally not present at landfill sites.
More recently Potts, Jr., et al., (U.S. Pat. No. 5,120,338) teach a method for separating a multi-component feedstream using distillation and a controlled freeze zone. This approach differs from the Ryan-Holmes process in that solid carbon dioxide is allowed to form in a controlled manner. This solid is melted and incorporated into the liquid portion of a liquid phase. A third gas phase is enriched in the most volatile component, methane, allowing its separation. By carefully controlling the conditions of solid formation, and gas-liquid distillation, the components may be separated into three streams. Essentially, this system allowed the desired product purity to be reached without avoiding the formation of solid carbon dioxide or the use of additives. The primary limitations of this process pertain to its scalability. The complexity and capital costs of the system require a biogas source larger than two million cubic feet per day to be economically viable. This approach is too complex and has too high of capital costs to be viable at smaller gas sources.
Several techniques are also taught that employ some cooling in conjunction with a second type of separation mechanism. For instance, Sweeney, et al. (U.S. Pat. No. 5,570,582), Soffer, et al. (U.S. Pat. No. 5,649,996), and Ojo, et al. (U.S. Pat. No. 5,531,808) teach processes by which the operation of adsorption systems is augmented by operation at sub-ambient or cryogenic temperatures. Lokhandwala (U.S. Pat. No. 5,647,227) teaches a process and apparatus by which a mixture of methane, nitrogen, and at least one other component (carbon dioxide) are separated. This processes employs a cryogenic separation augmented by a membrane. Such systems do not rely on solid phase formation or distillation to affect the separation. These hybrid systems also have costs and complexities which limit their use to landfill sites having biogas streams greater than approximately two million cubic feet per day.
In U.S. Pat. No. 5,642,630, Abdelmalek, et al. disclose a solid waste landfill gas treatment and separation process that claims production of a high quality liquefied natural gas stream, liquefied carbon dioxide stream, and a compressed natural gas stream. The patent teaches the use of a four-stage compressor to generate pressures up to 1800 psia, as well as three flash drums, the use of chemical additives, and multiple recirculation loops to obtain the desired products. The complexity of this system and related capital costs limit its usefulness at small landfill sites.
In U.S. Pat. No. 4,681,612 O'Brien, et al. disclose a cryogenic separation system that produces a fuel-grade methane product stream and the option of a carbon dioxide product stream. This approach relies on a cryogenic distillation column in which the methane is the more volatile, and thus the overhead product is enriched in methane. The methane is further separated from the overhead product with the use of a selective membrane. The bottom product primarily contains the carbon dioxide with impurities that may be further purified in a separate purification column and used as a product stream if desired. This approach suffers from two problems. First, because the system is a hybrid, using both a distillation column and a membrane, complexity and capital costs are increased. Second, high purity carbon dioxide and methane are not readily produced without subsequent processing and additional capital expenditures. Without the ability to produce high purity products, the applicability of this approach is limited.
Several techniques using chemical additives to separate carbon dioxide and methane in landfill gas and other gas streams have also been reported. Methanol is often used as a chemical additive (Apffel, U.S. Pat. No. 4,675,035). The addition of methanol to the gas mixture during the distillation decreases the temperature and pressure range at which solid carbon dioxide will form. This allows the distillation of the methane to proceed more completely, thereby providing higher purity products. Methanol can be separated from the carbon dioxide and recycled once the distillation process is complete. "Cold Methanol" separations as they are commonly called have offered one of the best methods to separate biogas to date. These systems, however, do not scale well to smaller biogas sources because of the system complexity, capital costs, and operating costs associated with the combined absorption and distillation processing equipment.
A second system employing chemical additives is taught by Abdelmalek (U.S. Pat. No. 5,642,630). This approach is one that uses chemical absorption to aid in the separation. As previously mentioned, systems requiring chemical additives and absorption increase operating costs due to the costs of the additive as well as capital costs and complexity to separate and recirculate the additives. Such systems are not economically feasible for biogas sources producing less than approximately two million standard cubic feet per day.
Broadly, it is the object of the present invention to provide an improved separation system process and apparatus for separating a gas stream containing, at a minimum, both carbon dioxide and methane, into high purity methane and high purity carbon dioxide product streams.
It is a yet further object of the present invention to provide a separation system that uses the formation of solid carbon dioxide to affect a highly effective separation.
It is a further object of the present invention to provide a separation system that has lower capital cost than prior art systems.
It is a still further object of the present invention to provide a separation system that is less complex than prior art systems. It is a yet further object of the present invention to provide a separation system that has lower operating costs than prior art systems.
These and other objects of the present invention will become apparent to those skilled in the art from the following detailed description of the invention and the accompanying drawings.